1. Field of the Invention
The present invention relates to a resin composition for coating which can form a coating film excellent in flexibility, hardness, stain resistance, etc.
2. Description of the Related Art
Alkyd resin type or polyester resin type coatings have excellent flexibility and are in wide use as coatings for steel plate materials, automobiles, industrial machinery, construction materials, cans for food and drink, etc. In recent years, however, the quality requirements for coatings have increased and it has become highly necessary to develop a coating capable of forming a coating film having excellent stain resistance, high hardness and good flexibility.
In coating films, however, (a) flexibility and (b) hardness and stain resistance are opposing properties, in general. As a coating resin satisfying all of these properties, there is proposed a polyester resin using terephthalic acid [Japanese Patent Application Kokai (Laid-Open) Nos. 57746/1982 and 117568/1982]; however, the resin is not fully satisfactory.
Acrylic resin type or vinyl resin type coatings are also known, but they are insufficient in film flexibility although they have a high film hardness.
Further, Japanese Patent Application Kokai (Laid-Open) No. 294778/1989 discloses, as a polyester resin coating, a coating comprising, as a main component, a polyester type graft polymer obtained by polycondensing polyester-forming components consisting mainly of a dibasic acid compound, a diol compound and a condensantion type macromonomer. In the polyester type graft polymer, however, when the macromonomer contains a (meth)acrylic acid ester, the alkyl group of the (meth)acrylic acid ester may cause dissociation during the polycondensation for production of the graft polymer, which may induce an esterification reaction starting from the dissociation site and may make it difficult to produce a desired graft polymer. Consequently, the resulting coating tends to give a coating film practically insufficient in hardness, chemical resistance, weather resistance, adhesion, etc.
Furthermore, it is known to synthesize a polyurethane graft polymer from a macromonomer having a dihydroxyl group at one end and a dissocyanate compound [POLYMER BULLETIN 8, 239-244 (1982)]. The polyurethane graft polymer, however, contains a large amount of urethane bond in the molecule, and consequently has a high structural viscosity and is poor in coatability, smoothness of coating film, etc.
In view of the above situation, the present inventors made study in order to develop a novel coating capable of forming a coating film excellent in flexibility, stain resistance and hardness. As a result, the present inventors found that a coating comprising, as main components, (A) a graft copolymer consisting of a particular backbone component and a particular branch component, wherein the two components are linked by urethane bond and (B) a polyisocyanate or an amino resin, can form a coating film satisfying all of the above-mentioned opposing properties [(a) flexibility and (b) hardness and stain resistance]. The present invention has been completed based on the finding.